Antarctic twilight

Video transcript

Welcome to Mawson station Antarctica! Now, a lot of my friends have said “you’ve told us the sun goes away for 15 days, but in your photos we see light! Why isn’t it completely dark?” There’s a good answer for that.

We all know that once the sun dips below the horizon, there’s that beautiful period of soft light where the sun is close enough to the horizon that the scattered rays of the sun still illuminate the sky and that’s what’s happening here at Mawson. There’s a long extended period where it’s just below the horizon within six degrees, for maybe about five hours even on the shortest day of the year. And that’s the twilight that we see!

An expeditioner watches as the sun appears over the horizon at Davis, for the first time in six weeks (Photo: Jeff R)

How much daylight is there in Antarctica during summer and winter?

On Antarctica’s coast, where our stations are located, there are usually a couple of weeks in mid-winter (around 21 June) when the sun does not rise, and a couple of weeks in summer around Christmas when there is 24-hour sunlight.

The polar circles (both the Antarctic Circle at 66°34′ S and Arctic Circle at 66°34′ N) mark the latitude beyond which the sun remains completely below the horizon throughout the day on Midwinter’s Day and completely above the horizon on Midsummer’s Day. As you move closer to the poles, the periods of winter darkness and summer daylight increase.

Compare the graphs below for Mawson and Davis. Davis is located further south than Mawson and therefore the sun does not rise here for a longer period of time during winter. At the poles themselves, the seasonal changes are even more pronounced: 24 hours of daylight occur for several months over summer, while there is complete darkness for several months during winter.

The diagrams below show how the length of day changes as you travel south to north from South Pole to Dome A, Davis, Mawson, Casey, Macquarie Island, Heard Island and Kingston, Tasmania.

Definitions

Civil twilight

This occurs after sunset or before sunrise when the centre of the sun is up to 6° below the horizon. At this time the brightest stars are normally visible, and under good weather conditions terrestrial objects will still be visible.

Nautical twilight

This occurs when the sun is between 6° and 12° below the horizon. At this time only the general outlines of ground objects may be visible, but the horizon is still visible, even on a moonless night.

Astronomical twilight

This occurs when the sun is between 12° and 18° below the horizon. There is no colour in the sky during astronomical twilight, and it is not possible to make out the horizon.